feat(icons): add brand icon picker from dashboard-icons

Add a second tab in the node Icon picker to choose from the ~2250 brand
icons hosted by homarr-labs/dashboard-icons (Plex, Sonarr, Home
Assistant, etc.) served via jsDelivr CDN. The Generic tab keeps the
existing lucide picker unchanged.

Storage uses a 'brand:<slug>' prefix on custom_icon, so existing nodes
referencing lucide keys keep working with zero migration. A new
resolveCustomIcon helper returns a discriminated union (lucide | brand)
and a NodeIcon component centralizes rendering for both kinds.

Includes a manifest fetch script (scripts/fetch-dashboard-icons.mjs)
and a checked-in dashboardIcons.json snapshot.

frontend/src/components/canvas/__tests__/BaseNode.test.tsx

@@ -44,6 +44,7 @@ vi.mock('@/utils/nodeColors', () => ({
 
 vi.mock('@/utils/nodeIcons', () => ({
   resolveNodeIcon: (_typeIcon: unknown) => _typeIcon,
+  isBrandIconKey: (k: string | undefined) => !!k && k.startsWith('brand:'),
 }))
 
 vi.mock('@/utils/maskIp', () => ({